Such reactors have a primary circuit in which the pressurized water for cooling the fuel assemblies of the reactor core circulates. The primary circuit communicates with the internal volume of the vessel containing the core and incorporates primary fluid circulating pumps, steam generators and a pressurizer, which are connected by means of large-diameter pressure-resistant pipes. The primary fluid of the reactor can likewise be circulated in certain auxiliary circuits, allowing it to be treated and its physical or chemical characteristics to be modified.
During its circulation in the primary circuit or in the auxiliary circuits, the cooling fluid comes in contact with many components, most of which are made of or covered with a nickel alloy making it possible to limit the extent to which they are attacked by the primary fluid. However, some components, such as the seats of valves and of cocks, or even some portions of piping, experience a certain wear, with the result that the primary fluid becomes laden with particles of very small dimensions which are torn off from these components. These particles tend to circulate with the primary fluid and therefore pass through the reactor core, where they are subjected to intense neutron bombardment, the effect of which is to activate them. In particular, wear-resistant alloys containing a certain proportion of cobalt cause highly activated particles to occur.
These particles accumulate in certain parts of the components of the reactor, thus presenting problems which are very difficult to solve during reactor maintenance operations, because these operations require preliminary decontamination phases which are very difficult to carry out.
On the other hand, the make-up water and the additives introduced into the primary fluid by means of an auxiliary circuit, such as the volumetric and chemical monitoring circuit, likewise contain solid particles of various origins which are activated when the primary fluid passes through the reactor core.
It is therefore necessary to treat the primary fluid periodically or continuously to reduce the content of activated or activatable particles in this primary fluid. These particles have a mean diameter of 0.5 microns with a considerable proportion of particles with a diameter of the order of 0.1 microns. These particles can also occur in colloidal form, i.e., in the form of a noncrystallized gel.
It is consequently necessary to purify the fluid by means of a process, such as ultrafiltration, more particularly by means of hot ultrafiltration, since the solubility of the pollutant products is greater when cold than when hot.
The idea was therefore to use hot-ultrafiltration processes on the primary fluid at its temperature and operating pressure in the reactor, to ensure its purification during the working of the reactor. Such a process is disclosed in French Pat. No. 2,552,419, which also discloses an ultrafiltration device which can be inserted in the circulation of the primary fluid inside the containment shell of the reactor. Such an ultrafiltration device, through which a fluid flows at a temperature in the neighborhood of 300.degree. and at a pressure of the order of 155.10.sup.5 Pa, has a pressure-resistant casing of very great thickness, made of a material resistant to the corrosive action of the primary fluid and its additives. The ultrafiltration wall consists of an assembly of composite tubes formed when homogeneous porous materials are superimposed on one another. One of the layers produced in microporous form serves as a separating membrane for the ultrafilter, the other layers constituting the support for the microporous layer. The tubes are fastened at each of their ends to a tube plate, the tube plates themselves being fastened to the inside of the casing. One of these tube plates corresponding to the inlet end of the tubes defines, together with the wall of the casing, an inflow chamber for the fluid to be purified, into which opens the pipe supplying this fluid. A filtrate recovery pipe opens into the casing between the two tube plates, and a concentrate discharge pipe is put into communication with the part of the casing receiving the concentrate, located between the tube-outlet tube plate and the end of the casing opposite the inflow chamber. The concentrate is kept in circulation to prevent the clogging of the ultrafiltration wall. During use over a long period of time, the tubes are liable to become clogged and the microporous layer deposited on the tubes may suffer wear which, in this case, can even go so far as to eliminate it completely. In any device of this type, it is therefore necessary to change the tubes after a certain period of use. In such filters, the tubes, integral with the tube plates which are themselves welded to the walls of the casing, cannot be replaced in a simple way, and it is consequently necessary to replace the filter as a whole when the tubes are clogged or worn.
On the other hand, the structure of the filter is such that, during rapid heat transience, differential expansions occur between the various parts of the filter, and this is detrimental to the mechanical strength of the filter and to ensuring good leak-proofing between the parts of this filter receiving the filtrate and the parts receiving the concentrate.
Finally, the volume of liquid circulating in the tubes decreases during filtration, because the filtrate passes through the wall of the tube, whereas the concentrate of decreasing volume continues to circulate in the tube. The rate of circulation of the fluid therefore tends to drop towards the end of the tubes, and there is consequently a risk of clogging towards the outlet end of the tubes.